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96                                            FLORA   IND1CA.
with the increasing latitude till the 30th degree north, the
cold of winter rapidly increases (see the map of Isothcr-
mals). Hence many tropical species, genera, and even families,
which are sensitive to cold, arc comparatively local when found
beyond the tropic, as most Palms, Cycas, Dipterocarpea (cx-
cept-Vatica), Aurantiace®, Connaracece, Meliacea, Myrtacete,
Rubiace&3 Ebenacea, and many more. Others arc indifferent
to the cold of winter, provided they experience a great sum-
mer heat; these advance far beyond the tropic, and lend a
more or less tropical aspect to the Flora even of the base of
the north-western Himalaya, in 33° north. Such are many
Legwninosa (as Bauhinia, Acada, Erythrinu, Butea, Dal-
bergia, Millettia], Bornbax, Vatica, Naticha, Combrctanw,.
Verbenacea, Lagerstrwnia, Gmfea, Jasmines, and liignonia
Indica.
Passing from the forest vegetation to that of annual plants,
we find that an immense proportion of these arc uniformly
distributed throughout India, and, vegetating only during
the hot rainy season, are neither exposed to drought nor cold.
Of these some of the most conspicuous arc*, besides Gratm-
nea and Cyperacea, a vast number of small LcyitmiitMtG and
Scropkiifarineff, Sida, Corchorus, Nauia, Bhunva and other
Composite) some Labiate (as IAWCM, A>tisomeh>s> etc,), Aaia»
ranthaccce, Acanlftacea, Convolvu/aww, Ludwiyia, Jmshmi^
etc,
Dr. Hoyle has well shown that this distribution of tropical
annuals and of pcrcumal-rootcA plants with annual shms is
not confined to the plains, but ascends the loftier mountain
valleys as far as the well-marked rainy season extends, nu<l
that s,uch plants only disappear where the accession of heat
and humidity is not sufficient in amount or regular enough
in period to stimulate their vegetative organs. Some of the
most remarkable of these oxtratropical examples of tropical
genera are species of "Begonia, Osfackiu, ArgMteMma,
trmthus, various Cyrttuufractw, ScUa»dneat Arawm,
tf, and u few epiphytal OrcAidets.